1. Field of the Invention
The present invention relates, in general, to a method for producing bioenergy with reduced discharge of carbon dioxide and process-related waste and, more particularly, to a method for producing zero-waste bioenergy with reduced emissions of carbon dioxide, i.e.: a representative greenhouse gas contributing to global warming; and maximized biomass treatment efficiency and bioenergy productivity.
2. Description of the Related Art
As well known in the art, examples of representative bioenergy sources may include fuels such as bioethanol and biodiesel. Until recently, bioethanol has been directly produced using yeast microorganisms such as Saccharomyces cerevisiae and Pichia stipites, which produce ethanol from saccharides such as glucose and xylose; or a microorganism such as enterobacter which produces ethanol from glycerol (Kim, J. H. et al., Ethanol production by simultaneous saccharification and fermentation with yeast Saccharomyces cerevisiae, The Korea Society for Energy Engineering, 10, 299-311, 2008).
Recently, biobutanol has been actively studied as a next generation fuel along with bioethanol, and has been highlighted as the next generation fuel due to its lack of engine corrosion and high miscibility with gasoline at high concentration (Bahl, H. W. et al., continuous production of acetone and butanol by Clostridium acetobutylicum in a two-stage phosphate limited chemostat, European Journal of Applied Microbiology and Biotechnology, 15, 201-205, 1982).
Biobutanol may be produced by butanol-producing microorganisms such as Clostridium acetobutylicum, Clostridium beijerinckii, and Clostridium saccharobutylicum; or directly from Clostridium pasteurianum, etc., which produce butanol from glycerol, and which also produce 1,3-propanediol (Ezeji, T. N. et al., Butanol production from agricultural residues: impact of degradation products on Clostridium beijerinckii growth and Butanol fermentation, Biotechnology and Bioengineering, 97, 1460-1469, 2007).
The amount of the global bioethanol production, which had reached 46 billion liters in 2005, has been continuously increasing (Jae, J. K. et al., A review on thermochemical pretreatment in Lignocellulosic bioethanol production, Korea Organic Resource Recycling Association, 16, 79-88, 2008). However, since the resulting production of the fermentation liquid waste, corresponding to about 6-10 times that of the bioethanol production, is a source of environmental pollution at the time of its release, the technology of treating the fermentation liquid waste will soon become a core, essential technology in the bioethanol production process.
Meanwhile, an anaerobic digestion process is a process utilizing a series of sequential microbial reactions where organic wastes are decomposed under anaerobic conditions, and a biogas containing methane and carbon dioxide is produced in the final step. In particular, since the advent of methane, a major component of a biogas, is a suitable energy source, the anaerobic digestion process has been widely used in energy production as well as in treating: excess sludge generated in sewage treatment plants, various kinds of byproducts generated in the food industry, high concentration organic wastewater, etc. Biogas refers to a gas generated during the fermentation/decomposition of organic waste under anaerobic conditions, including anaerobic digestion gas (ADG) and landfill gas (LFG). The anaerobic digestion gas refers to a gas generated during the anaerobic digestion process of organic materials such as food waste, livestock manure, and sewage sludge; and a landfill gas refers to a gas generated in landfill sites.
Biogas typically consists in CH4 (40-60%), CO2 (30-45%), H2S (0.1-5%), N2 (2-5%), and O2 (0.1-1%) as major components, along with a trace amount of other components including: CO, H2, NH3, mercaptan, and VOCs. In particular, the representative biogas components of CH4 may be used as energy sources for co-generation plants, gas boilers, heating and cooling systems, heat pumps, etc The calorific value of pure methane gas is about 9,000 kcal/m3 and that of unpurified biogas is about 5,000-7,000 kcal/m3. Accordingly, biogas has been highlighted as a major source of substitute energy due to recent high oil prices with the corresponding increasing fossil-fuel prices.
In the present invention, microalgae may refer to all single-celled and multi-celled microorganisms which belong to prokaryotic and eukaryotic algae, and cyanobacteria. Here, the term microalgae is further sub-divided into autotrophic-, heterotrophic-, and mixotrophic algae.
Autotrophic microalgae perform a so-called autotrophic energy metabolism when CO2 is present as a carbon source during cultivation; during which CO2 is reduced to carbohydrates, e.g.: starch in green algae and glycogen in blue-green algae; thereby being converted into an algal biomass as.6CO2+12H2O+(light energy)->C6H12O6+6O2+6H2O
Additionally, autotrophic microalgae, unlike the heterotrophic bacteria in conventional biological processes or in biofilers, can store energy in biomass via taking up CO2 from biogas and CO2-dissolved inorganic carbons without supplying additional carbon source. Therefore, the increase in microalgal biomass becomes final product of CO2 elimination. In contrast, heterotrophic microalgae may be cultivated in stirred biological reactors or fermenters using organic carbon substrate, such as glucose, as a carbon source. Mixotrophic microalgae can perform both autotrophic and heterotrophic energy metabolism for growth.
Furthermore, when the microalgal biomass which is produced during the removal of CO2 from biogas has a higher triglyceride content than that of other microorganisms, the microalgal biomass may be used as a feedstock for producing biodiesel. When microalgae is cultivated at high concentration, the resulting oil yield per unit area will be high compared to other crops and biomass can be produced at a faster rate than the general oil-producing crops. The doubling time of microalgae during exponential growth under optimal environment is a few hours at the shortest and within 2 or 3 days at the longest. Since microalgae can be cultivated in indoor bioreactors instead of outdoors, the production yield and productivity of microalgae can be improved via cultivation engineering. Accordingly, in a smaller country like South Korea where acquiring large cultivation area is difficult and weather condition is not optimal for biodiesel-producing crops; it is necessary to produce biodiesel from microalgle biomass in an indoor, intensive, high-density cultivation facility.
Furthermore, biodiesel in the form of fatty acid alkyl ester is produced through extracting fats/oil from cultivated algae via physical, chemical, and biological methods; and then carrying out trans-esterification of the extracted fats/oil. Therefore, fatty acid methyl ester (FAME) is produced as biodiesel when methanol is used for the reaction as below.

Bioenergy, being ‘carbon neutral’, can contribute to delaying and preventing global warming by reducing carbon dioxide emissions, compared to the conventional petroleum energy. Biosaccharification/ethanol fermentation, biosaccharification/butanol fermentation and biodiesel production are well-known bioenergy production processes.
Unlike other renewable energies, bioenergy in the liquid form like bioethanol and biodiesel can be mainly used as transportation fuel. energy as is the case with bioethanol and biodiesel.
As for the raw materials for producing bioenergy, starch such as edible corn and vegetable oil have been used as the first generation of biomass for producing bioethanol and biodiesel, respectively. As the second generation biomass, inedible fruit pulp, herbaceous plants, macroalgae, etc., have been used as raw materials for producing bioethanol, and also inedible freshwater or marine microalgae, waste vegetable oil or inedible vegetable oil such as rapeseed oil have been used. Recently, biomass such as woody plant and aerobic/anaerobic sludge has been spotlighted.
In South Korea, it is not possible to stably supply biomass. In the present invention, macroalgae such as Sargassum, Gracilaria, Prymnesium parvum, Euglena gracilis, Gelidium amansii, Laminaria, etc., may be used. However, under the current coastal conditions of South Korea, cultivating macroalgae for their continuous and stable supply is not easy. Additionally, the rape flower cultivation as a raw material for producing rapeseed oil has been reported uneconomical in South Korea. Furthermore, the lignocellulosic biomass has been mainly obtained from the ‘forest tending’ project performed two or three times annually or sawdust, and the transportation cost from biomass collection to users is considerably high as compared with other renewable energy sources.